Automatic tone correction apparatus, automatic tone correction method, and automatic tone correction program storage mediums Number:7,142,712 from the United States Patent and Trademark Office (PTO) owispatent An automatic tone correction apparatus comprises a luminance distribution formation circuit for forming a luminance histogram of an input image; a center-of-gravity calculation circuit for calculating the center of gravity of a low-luminance part of the luminance histogram; a distribution range calculation circuit for calculating a maximum value and a minimum value on an input luminance level axis; a target value storage circuit for holding a luminance correction target value; a correction table formation circuit for forming a correction table for expanding the luminance distribution and, simultaneously, correcting the luminance histogram so that the center of gravity matches the luminance correction target value; and an image signal correction circuit for performing tone correction on the basis of the correction table. Therefore, whether the input image is a forward-light image or a back-lighted image, the apparatus performs tone correction using the same processing.<H correction, automatic, Automatic, tone, , 7142712.html, Patent, pto, 7142712

Title: Automatic tone correction apparatus, automatic tone correction method, and automatic tone correction program storage mediums

Abstract: An automatic tone correction apparatus comprises a luminance distribution formation circuit for forming a luminance histogram of an input image; a center-of-gravity calculation circuit for calculating the center of gravity of a low-luminance part of the luminance histogram; a distribution range calculation circuit for calculating a maximum value and a minimum value on an input luminance level axis; a target value storage circuit for holding a luminance correction target value; a correction table formation circuit for forming a correction table for expanding the luminance distribution and, simultaneously, correcting the luminance histogram so that the center of gravity matches the luminance correction target value; and an image signal correction circuit for performing tone correction on the basis of the correction table. Therefore, whether the input image is a forward-light image or a back-lighted image, the apparatus performs tone correction using the same processing.

Patent Number: 7,142,712 Issued on 11/28/2006 to Maruoka, et al.

Inventors: Maruoka; Kazutaka (Saijyo, JP), Fujimura; Fumio (Saijyo, JP)
Assignee: Matsushita Electric Industrial Co., Ltd (Osaka, JP)

Appl. No.: 10/170,626

Filed: June 14, 2002

Foreign Application Priority Data


Jun 14, 2001 [JP]			2001-180642

Current U.S. Class: 382/168 ; 358/1.9

Current International Class: G06K 9/00 (20060101)

Field of Search: 382/162-169,115-123,288 358/518-522,1.9,3.26 348/77-78,576-577 345/589-604

References Cited [Referenced By]

U.S. Patent Documents


5982926	November 1999	Kuo et al.
6631208	October 2003	Kinjo et al.
6873441	March 2005	Kuwabara et al.

Foreign Patent Documents


04-035467	Feb., 1992	JP
05-205033	Aug., 1993	JP
6-268866	Sep., 1994	JP
09-200531	Jul., 1997	JP
10-243261	Sep., 1998	JP
11-317872	Nov., 1999	JP
2000-102033	Apr., 2000	JP
2000-236452	Aug., 2000	JP
2000-278530	Oct., 2000	JP
2001-94804	Apr., 2001	JP

Primary Examiner: Sherali; Ishrat
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.

Claims

What is claimed is:

1. An automatic tone correction apparatus comprising: a luminance histogram formation unit for forming a luminance histogram of a digital image; a center-of-gravity calculation unit for calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and performing tone correction using the correction table, thereby expanding the luminance distribution and, simultaneously, matching the center of gravity of the low-luminance part of the luminance histogram to the luminance correction target value.

2. An automatic tone correction apparatus as defined in claim 1, wherein a restriction is put on the amount of correction in the tone correction.

3. An automatic tone correction apparatus as defined in claim 1, wherein the luminance correction target value to be used in the tone correction is previously set according to the position of the center of gravity.

4. An automatic tone correction apparatus comprising: a luminance histogram formation unit for forming a luminance histogram of a digital image; a target value calculation unit for calculating a luminance correction target value from a target luminance distribution; a center-of-gravity calculation unit for calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and the luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit.

5. An automatic tone correction apparatus as defined in claim 4 wherein, when performing the tone correction, the target value calculation unit sets a target luminance distribution with a curve.

6. An automatic tone correction apparatus as defined in claim 4 wherein, when performing the tone correction, the target value calculation unit sets a plurality of target luminance distributions, and stores them.

7. An automatic tone correction apparatus comprising: a luminance histogram formation unit for forming a luminance histogram of a digital image; a boundary value calculation unit for calculating a boundary value for determining a low-luminance part of the luminance histogram; a center-of-gravity calculation unit for calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated by the boundary value calculation unit; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit.

8. An automatic tone correction apparatus as defined in claim 7 therein, when performing the tone correction, the boundary value calculation unit divides a luminance range into plural regions, and calculates a boundary value from the feature parameters of the respective regions.

9. An automatic tone correction apparatus as defined in claim 8 wherein, when performing the tone correction, the boundary value calculation unit divides the luminance range into two regions, and a center of gravity calculated on the lower luminance side is used as the center of gravity of the low-luminance part of the luminance histogram.

10. An automatic tone correction apparatus comprising: a luminance histogram formation unit for forming a luminance histogram of a digital image; a mountain-shape distribution detection unit for detecting a range of a mountain-shape distribution of the luminance histogram; a boundary value calculation unit for calculating a boundary value for determining a low-luminance part of the luminance histogram from the range of the mountain-shape distribution detected by the mountain-shape distribution detection unit; a center-of-gravity calculation unit for calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated by the boundary value calculation unit; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit.

11. An automatic tone correction apparatus as defined in claim 10 wherein, when performing the tone correction, the boundary value calculation unit sets the boundary value calculated from the range of the mountain-shape distribution, using the luminance histogram.

12. An automatic tone correction apparatus as defined in claim 10 wherein, when performing the tone correction, the mountain-shape distribution detection unit puts a restriction on the range of the mountain-shape distribution.

13. An automatic tone correction apparatus as defined in any of claims 1 to 12 wherein, when performing the tone correction, the distribution of the luminance histogram is expanded only toward the low-luminance side and, simultaneously, the luminance histogram is corrected so that the center of gravity of the luminance histogram matches the luminance correction target value.

14. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein the tone correction unit performs the tone correction by executing .gamma. correction.

15. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein the tone correction unit forms the correction table so that it contains a correction which performs, simultaneously, a process of expanding the luminance distribution of the luminance histogram, and a process of matching the center of gravity of the low-luminance part of the distribution-expanded luminance histogram to the luminance correction target value.

16. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, further comprising a luminance distribution expansion unit for expanding the luminance distribution of the luminance histogram formed by the luminance histogram formation unit; wherein the center-of-gravity calculation unit calculates the center of gravity of the low-luminance part of the luminance histogram in which the luminance distribution is expanded.

17. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein, when forming the luminance histogram, a restriction is put on the amount of accumulation of luminance distribution.

18. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, further comprising a skin color region extraction unit for extracting a skin color region from color-difference signals and a luminance signal; wherein, when forming the luminance histogram, the luminance distribution of a skin color portion of the image is added to the luminance distribution of the whole image according to the area of the skin color region.

19. An automatic tone correction apparatus as defined in claim 18 wherein, when the luminance distribution of the skin color portion of the image is added to the luminance distribution of the whole image according to the area of the skin color region, the distribution of a high-luminance part of the skin color portion is weighted.

20. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein the center of gravity of the high-luminance part is calculated from the luminance histogram which is calculated by the luminance histogram formation unit, and an amount of tone correction on the high-luminance part is calculated according to the position of the center of gravity of the high-luminance part.

21. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein tone correction is not carried out when the luminance distribution is discrete.

22. An automatic tone correction apparatus as defined in any of claims 1, 4, 7, and 10, wherein color-difference signals of the image are corrected according to the correction table.

23. An automatic tone correction method for correcting tones of a digital image, comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

24. An automatic tone correction method as defined in claim 23 wherein, in the tone correction step, a restriction is put on the amount of correction.

25. An automatic tone correction method as defined in claim 23, wherein the luminance correction target value to be used in the tone correction step is previously set according to the position of the center of gravity.

26. An automatic tone correction method for correcting tones of a digital image, comprising: a target value calculation step of calculating a luminance correction target value from a target luminance distribution; a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value that is calculated in the target value calculation step.

27. An automatic tone correction method as defined in claim 26 wherein, in the target value calculation step, a target luminance distribution is set with a curve.

28. An automatic tone correction method as defined in claim 26 wherein, in the target value calculation step, a plurality of target luminance distributions are stored.

29. An automatic tone correction method for correcting tones of a digital image, comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a boundary value calculation step of calculating a boundary value for setting a range of a low-luminance part of the luminance histogram; a center-of-gravity calculation step of calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

30. An automatic tone correction method as defined in claim 29 wherein, in the boundary value calculation step, a luminance range is divided into plural regions, and a boundary value is calculated from feature parameters of the respective regions.

31. An automatic tone correction method as defined in claim 30 wherein, in the boundary value calculation step, the luminance range is divided into two regions, and a center of gravity calculated on the lower-luminance side is used as the center of gravity of the low-luminance part of the luminance histogram.

32. An automatic tone correction method for correcting tones of a digital image, comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a mountain-shape distribution detection step of detecting a range of a mountain-shape distribution of the luminance histogram; a boundary value calculation step of calculating a boundary value for setting a low-luminance part of the luminance histogram, from the range of the mountain-shape distribution detected in the mountain-shape distribution detection step; a center-of-gravity calculation step of calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

33. An automatic tone correction method as defined in claim 32 wherein, in the boundary value calculation step, the boundary value calculated from the range of the mountain-shape distribution is set by using the luminance histogram.

34. An automatic tone correction method as defined in claim 32 wherein, in the mountain-shape distribution detection step, a restriction is put on the range of the mountain-shape distribution.

35. An automatic tone correction method as defined in any of claims 23 to 34, wherein the distribution of the luminance histogram is expanded only toward the lower-luminance side and, simultaneously, the luminance histogram is corrected so that the center of gravity of the luminance histogram matches the luminance correction target value.

36. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein the tone correction step is performed by executing .gamma. correction.

37. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein, in the tone correction step, a correction table is formed so that it contains a correction which performs, simultaneously, a process of expanding the luminance distribution of the luminance histogram, and a process of matching the center of gravity of the low-luminance part of the distribution-expanded luminance histogram to the luminance correction target value.

38. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, further comprising a luminance distribution expansion step of expanding the luminance distribution of the luminance histogram, between the luminance histogram formation step and the center-of-gravity calculation step; wherein, in the center-of-gravity calculation step, calculation of a center of gravity is performed on the low-luminance part of the luminance histogram in which the luminance distribution is expanded.

39. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein, when forming the luminance histogram, a restriction is put on the amount of accumulation of luminance distribution.

40. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, further comprising a skin color region extraction step of extracting a skin color region from color-difference signals and a luminance signal; wherein, in the luminance histogram formation step, a luminance histogram, in which the luminance distribution of a skin color part of the image is added to the luminance distribution of the whole image according to the area of the skin color region, is formed.

41. An automatic tone correction method as defined in claim 40 wherein, when the luminance distribution of the skin color part of the image is added to the luminance distribution of the whole image according to the area of the skin color region, the distribution of a high-luminance portion of the skin color part is weighted.

42. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein the center of gravity of the high-luminance part is calculated from the luminance histogram which is calculated in the luminance histogram formation step, and an amount of tone correction for the high-luminance part is calculated according to the position of the center of gravity of the high-luminance part.

43. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein, in the tone correction, tone correction is not carried out when the luminance distribution is discrete.

44. An automatic tone correction method as defined in any of claims 23, 26, 29, and 32, wherein color-difference signals of the image are corrected according to the correction table.

45. An automatic tone correction program storage medium readable by computer in which a program for making a computer execute an automatic tone correction method for correcting tones of a digital image is stored, said automatic tone correction method comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

46. An automatic tone correction program storage medium readable by computer in which a program is stored for making a computer execute an automatic tone correction method for correcting tones of a digital image is stored, said automatic tone correction method comprising: a target value calculation step of calculating a luminance correction target value from a target luminance distribution; a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value that is calculated in the target value calculation step.

47. An automatic tone correction program storage medium readable by computer in which a program is stored for making a computer execute an automatic tone correction method for correcting tones of a digital image is recorded, said automatic tone correction method comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a boundary value calculation step of calculating a boundary value for setting a range of a low-luminance part of the luminance histogram; a center-of-gravity calculation step of calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

48. An automatic tone correction program storage medium readable by computer in which a program is stored for making a computer execute an automatic tone correction method for correcting tones of a digital image is stored, said automatic tone correction method comprising: a luminance histogram formation step of forming a luminance histogram of the digital image; a mountain-shape distribution detection step of detecting a range of a mountain-shape distribution of the luminance histogram; a boundary value calculation step of calculating a boundary value for setting a low-luminance part of the luminance histogram, from the range of the mountain-shape distribution detected in the mountain-shape distribution detection step; a center-of-gravity calculation step of calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value.

49. An automatic tone correction program storage medium as defined in any of claims 45 to 48 wherein, in the tone correction step, a correction table is formed so that it contains a correction which performs, simultaneously, a process of expanding the luminance distribution of the luminance histogram, and a process of matching the center of gravity of the low-luminance part of the distribution-expanded luminance histogram to the luminance correction target value.

50. An automatic tone correction program storage medium as defined in any of claims 45 to 48, wherein the automatic tone correction method further comprises a luminance distribution expansion step of expanding the luminance distribution of the luminance histogram, between the luminance histogram formation step and the center-of-gravity calculation step; and in the center-of-gravity calculation step, calculation of a center of gravity is performed on the low-luminance part of the luminance histogram in which the luminance distribution is expanded.

51. An automatic tone correction program storage medium as defined in any of claims 45 to 48 wherein, when forming the luminance histogram, a restriction is put on the amount of accumulation of luminance distribution.

52. An automatic tone correction program storage medium as defined in any of claims 45 to 48, wherein the automatic tone correction method further comprises a skin color region extraction step of extracting a skin color region from color-difference signals and a luminance signal; and in the luminance histogram formation step, a luminance histogram, in which the luminance distribution of a skin color part of the image is added to the luminance distribution the whole image according to the area of the skin color region, is formed.

53. An automatic tone correction program storage medium as defined in claim 52 wherein, when the luminance distribution of the skin color part of the image is added to the luminance distribution of the whole image according to the area of the skin color region, the distribution of a high-luminance portion of the skin color part is weighted.

54. An automatic tone correction program storage medium as defined in any of claims 45 to 48, wherein the center of gravity of the high-luminance part is calculated from the luminance histogram which is calculated in the luminance histogram formation step, and an amount of tone correction for the high-luminance part is calculated according to the position of the center of gravity of the high-luminance part.

55. An automatic tone correction program storage medium as defined in any of claims 45 to 48, wherein tone correction is not carried out when the luminance distribution is discrete.

56. An automatic tone correction program storage medium as defined in any of claims 45 to 48, wherein color-difference signals of the image are corrected according to the correction table.

Description

FIELD OF THE INVENTION

The present invention relates to automatic tone correction apparatuses, automatic tone correction methods, and automatic tone correction program storage mediums. To be specific, the invention relates to an improved digital image processing apparatus that can correct an input image, more specifically, that can automatically correct tones of an input image.

BACKGROUND OF THE INVENTION

As a conventional technique for automatically correcting tones of an image, a method as follows has been well known. That is, a luminance value or a density value of each pixel of an input image is calculated, a histogram indicating the sum of pixels having the same value is formed, and all pixels are subjected to such a correction process that optimizes the shape of the histogram.

For example, Japanese Published Patent Application No. Hei.6-268866 discloses a method for performing correction using a histogram as mentioned above. The method disclosed in this literature will be described with reference to FIGS. 40(a) 40(c). In the following description, it is assumed that each pixel of a digital image has a density value ranging from "0" to "255".

Initially, a density value of each pixel is calculated from a digital image signal, and a density histogram indicating the sum of pixels having the same density value is formed. An example of this histogram is shown in FIG. 40(a). Next, assuming that the minimum density value of the created density histogram is dmin, the maximum density value is dmax, the number of pixels having a pixel density d is F1[d], and the number of pixels after conversion is F2[d], a histogram which is corrected so as to expand a pixel density distribution area over the whole pixel luminance (refer to FIG. 40(b)) is formed according to formula (1) as follows and, simultaneously, a point (center of gravity) G which divides the area of the histogram into two equal parts is calculated. Thereafter, as shown in FIG. 40(c), a .gamma. value at which the center of gravity G is positioned in the center of the density range is obtained. Assuming that the number of pixels after conversion of the F2[d] is expressed by F3[d], a histogram which is subjected to .gamma. correction is formed according to formula (2) as follows, and correction is carried out on the basis of the histogram. F2[d]=F1[d.times.{(dmax-dmin)/255+dmin}] (1)

.function..gamma. ##EQU00001##

In this way, the distribution of pixel densities is expanded over all tones, evenly from the center tone to the both sides, whereby the brightness is adjusted so that the center of gravity of the density distribution is positioned in the center.

Meanwhile, Japanese Published Patent Application No. 2000-102033 discloses another method for performing tone correction on an image using a histogram. This method will be described with reference to FIG. 41.

In this method, an input luminance level axis, which is calculated for a luminance histogram generated from a digital image signal, is divided into equal n regions (n: integer that satisfies n>3), and feature parameters, such as a ratio of the number of pixels in each region to all pixels, a ratio of the number of pixels in each region exceeding a predetermined limit value to all pixels, and a radio of the number of pixels in each of three equal regions into which the input luminance-level axis is divided, are captured (step 2501). Next, various kinds of curve data which have previously been formed are captured (step 2502). Then, the process is branched into different processes (case branching) in step 2503 and step 2505 on the basis of the feature parameters, and a tone correction curve is formed from the feature parameters and curve data, which are captured in steps 2501 and 2502, by under processing (step 2504), over processing (step 2506), or linear processing (step 2507), and tone correction curve data is stored (step 2508). Then, tone correction is carried out on the basis of the tone correction curve so obtained.

That is, in this method, the correction process is branched into different processes according to the shape of the luminance histogram of the input image, and a tone correction curve is formed by a process most suited to the input image, and then tone correction is carried out on the basis of the tone correction curve.

As described above, according to the tone correction method disclosed in Japanese Published Patent Application No. Hei.6-268866, even when an input image includes a lot of halftones, a sharp-contrast image can be obtained.

On the other hand, according to the tone correction method disclosed in Japanese Published Patent Application No. 2000-102033, an image of improved quality is obtained as compared with the case of using the accumulated luminance distribution as it is. Further, since the correction process can be implemented by software, it can easily deal with an alteration of image data or the like.

However, the tone correction method disclosed in Japanese Published Patent Application No. Hei.6-268866 has a drawback as follows. Although, in this method, .gamma. correction is carried to bring the center of gravity of the histogram to the center of tones, when two peaks are generated on the low-tone side and the high-tone side of the histogram as in a back-lighted imager correction is hardly carried out because the center of gravity of the histogram is originally positioned in the vicinity of the center tone. Consequently, the back-lighted area of the image is not corrected at all.

Furthermore, as for a natural image such as a scenic shot, a natural image having a histogram the center of gravity of which is in the vicinity of the center tone is not always beautiful. For example, when an image including a person who wears black clothes is corrected, the clothes unfavorably turn to gray.

On the other hand, in the tone correction method disclosed in Japanese Published Patent Application No. 2000-102033, appropriate tone correction is carried out by branching the correction process according to the shape of the luminance histogram of the input image. In this method, however, accurate case branching is required, whereby the process is complicated and the volume of calculation is increased.

Moreover, different correction processes must be prepared for the respective cases, whereby the required storage capacity is increased, and the scale of the correction apparatus is increased, resulting in an increase in costs.

Furthermore, in the above-described methods, when a person exists in the center of the image and the area occupied by the person is small, since information of the person is hardly included in the histogram, an unintended correction is carried out, whereby a skin color region such as a face, which is visually conspicuous, occurs halation.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-described problems and has for its object to provide an automatic tone correction apparatus, an automatic tone correction method, and an automatic tone correction program storage medium, that can realize satisfactory correction for a back-lighted image without collapsing a high-luminance part of the image, that can optimize the contrast and brightness of a forward-lighted image by the same processing as that for the back-lighted image, and that can suppress unintended correction.

Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.

According to a 1st aspect of the present invention, an automatic tone correction apparatus comprises: a luminance histogram formation unit for forming a luminance histogram of a digital image; a center-of-gravity calculation unit for calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and performing tone correction using the correction table, thereby expanding the luminance distribution and, simultaneously, matching the center of gravity of the low-luminance part of the luminance histogram to the luminance correction target value. Therefore, a back-lighted image can be brightly corrected so that the luminance value of a low-luminance region in the back-lighted state is brought to a target luminance value, thereby obtaining an image the whole contrast of which is enhanced Further, also a forward-lighted image can be corrected so that the luminance value of a low-luminance region is brought to a target luminance value, resulting in an image the whole contrast of which is enhanced.

According to a 2nd aspect of the present invention, in the automatic tone correction apparatus according to the 1st aspect, a restriction is put on the amount of correction in the tone correction. Therefore, even when the luminance correction target value is not an optimum value, the property of the original image is prevented from being damaged significantly.

According to a 3rd aspect of the present invention, in the automatic tone correction apparatus according to the first aspect, the luminance correction target value to be used in the tone correction is previously set according to the position of the center of gravity. Therefore, a work of setting a luminance correction target value for every input image is dispensed with.

According to a 4th aspect of the present invention, an automatic tone correction apparatus comprises: a luminance histogram formation unit for forming a luminance histogram of a digital image; a target value calculation unit for calculating a luminance correction target value from a target luminance distribution; a center-of-gravity calculation unit for calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and the luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit. Therefore, an image having a target brightness and an enhanced contrast as a whole can be obtained using a target luminance distribution of the same shape for a forward-lighted image and a back-lighted image.

According to a 5th aspect of the present invention, in the automatic tone correction apparatus according to the 4th aspect, when performing the tone correction, the target value calculation unit sets a target luminance distribution with a curve. Therefore, setting of a target luminance distribution is facilitated, whereby the storage capacity is reduced.

According to a 6th aspect of the present invention, in the automatic tone correction apparatus according to the 4th aspect, when performing the tone correction, the target value calculation unit sets a plurality of target luminance distributions, and stores them. Therefore, the luminance correction target value can be easily adjusted.

According to a 7th aspect of the present invention, an automatic tone correction apparatus comprises: a luminance histogram formation unit for forming a luminance histogram of a digital image; a boundary value calculation unit for calculating a boundary value for determining a low-luminance part of the luminance histogram; a center-of-gravity calculation unit for calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated by the boundary value calculation unit; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit. Therefore, in a back-lighted image, a low-luminance part including a back-lighted region is correctly calculated, whereby the luminance value of the back-lighted region is corrected to a target luminance value more accurately.

According to an 8th aspect of the present invention, in the automatic tone correction apparatus according to the 7th aspect, when performing the tone correction, the boundary value calculation unit divides a luminance range into plural regions, and calculates a boundary value from the feature parameters of the respective regions. Therefore, a boundary value that accurately separates the high-luminance side and the low-luminance side is obtained.

According to a 9th aspect of the present invention, in the automatic tone correction apparatus according to the 8th aspect, when performing the tone correction, the boundary value calculation unit divides the luminance range into two regions, and a center of gravity calculated on the lower luminance side is used as the center of gravity of the low-luminance part of the luminance histogram. Therefore, the tones can be corrected without recalculating the center of gravity of the low-luminance part.

According to a 10th aspect of the present invention, an automatic tone correction apparatus comprises: a luminance histogram formation unit for forming a luminance histogram of a digital image; a mountain-shape distribution detection unit for detecting a range of a mountain-shape distribution of the luminance histogram; a boundary value calculation unit for calculating a boundary value for determining a low-luminance part of the luminance histogram from the range of the mountain-shape distribution detected by the mountain-shape distribution detection unit; a center-of-gravity calculation unit for calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated by the boundary value calculation unit; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and a luminance correction target value that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value calculated by the target value calculation unit. Therefore, when the input image is a back-lighted image, it is corrected so as to have a target brightness and, simultaneously, the whole contrast is enhanced. On the other hand, when the input image is a forward-lighted image, the contrast is enhanced without changing the original brightness.

According to an 11th aspect of the present invention, in the automatic tone correction apparatus according to the 10th aspect, when performing the tone correction, the boundary value calculation unit sets the boundary value calculated from the range of the mountain-shape distribution, using the luminance histogram. Therefore, a more accurate boundary value can be calculated.

According to a 12th aspect of the present invention, in the automatic tone correction apparatus according to the 10th aspect, when performing the tone correction, the mountain-shape distribution detection unit puts a restriction on the range of the mountain-shape distribution. Therefore, even when the input image is a forward-lighted image, the brightness of the image can be corrected.

According to a 13th aspect of the present invention, an automatic tone correction apparatus comprises: a luminance histogram formation unit for forming a luminance histogram of a digital image; a center-of-gravity calculation unit for calculating a center of gravity of a low-luminance part of the luminance histogram; and a target value correction unit for correcting a luminance correction target value, that is a target value to which the center of gravity of the low-luminance part of the luminance histogram should be shifted, so that the target value matches the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, when the target value is smaller than the center of gravity of the low-luminance part of the luminance histogram; and a tone correction unit for forming a correction table on the basis of the luminance histogram which is formed by the luminance histogram formation unit, the center of gravity of the low-luminance part of the luminance histogram which is calculated by the center-of-gravity calculation unit, and the luminance correction target value, and using the correction table, expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value which is corrected or not corrected by the target value correction unit. Therefore, it is prevented that the corrected image becomes darker than the input image.

According to a 14th aspect of the present invention, in the automatic tone correction apparatus according to the 13th aspect, when performing the tone correction, if the luminance correction target value is larger than the center of gravity of the low-luminance part of the luminance histogram, the target value correction unit corrects the luminance correction target value so that the target value matches the center of gravity of the low-luminance part of the luminance histogram. Therefore, it is prevented that the corrected image becomes brighter than the input image.

According to a 15th aspect of the present invention, in the automatic tone correction apparatus according to the 13th or 14th aspect, when performing the tone correction, the luminance correction target value is corrected so that the target value becomes a value in the vicinity of the center of gravity of the low-luminance part of the luminance histogram. Therefore, the corrected brightness can be minutely adjusted on the basis of the brightness of the input image.

According to a 16th aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st to 14th aspects, when performing the tone correction, the distribution of the luminance histogram is expanded only toward the low-luminance side and, simultaneously, the luminance histogram is corrected so that the center of gravity of the luminance histogram matches the luminance correction target value. Therefore, tone correction can be carried out without collapsing the high-luminance part.

According to a 17th aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, the tone correction unit performs the tone correction by executing .gamma. correction. Therefore, matching of the center of gravity of the low-luminance part of the luminance histogram to the target value is realized by just once formation of a conversion table.

According to an 18th aspect of the present invention, in the go automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, the tone correction unit forms the correction table so that it contains a correction which performs, simultaneously, a process of expanding the luminance distribution of the luminance histogram, and a process of matching the center of gravity of the low-luminance part of the distribution-expanded luminance histogram to the luminance correction target value. Therefore, matching of the center of gravity of the low-luminance part of the luminance histogram to the target value is realized by just once formation of a conversion table.

According to a 19th aspect of the present invention, the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects further comprises a luminance distribution expansion unit for expanding the luminance distribution of the luminance histogram formed by the luminance histogram formation unit; and the center-of-gravity calculation unit calculates the center of gravity of the low-luminance part of the luminance histogram in which the luminance distribution is expanded. Therefore, automatic tone correction is realized by performing the process of matching the center of gravity of the low-luminance part to the target value, after expanding the luminance distribution of the luminance histogram.

According to a 20th aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, when forming the luminance histogram, a restriction is put on the amount of accumulation of luminance distribution. Therefore, even when the tone distribution of the luminance histogram concentrates on a certain portion, the amount of correction can be accurately calculated.

According to a 21st aspect of the present invention, the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects further comprises a skin color region extraction unit for extracting a skin color region from color-difference signals and a luminance signal; and when forming the luminance histogram, the luminance distribution of a skin color portion of the image is added to the luminance distribution of the whole image according to the area of the skin color region. Therefore, when a person exists in the image, tone correction can be carried out so that the brightness of the skin color is optimized according to the area of the person.

According to a 22nd aspect of the present invention, in the automatic tone correction apparatus according to the 21st aspect, when the luminance distribution of the skin color portion of the image is added to the luminance distribution of the whole image according to the area of the skin color region, the distribution of a high-luminance part of the skin color portion is weighted. Therefore, halation of the skin color is minimized.

According to a 23rd aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, the center of gravity of the high-luminance part is calculated from the luminance histogram which is calculated by the luminance histogram formation unit, and an amount of tone correction on the high-luminance part is calculated according to the position of the center of gravity of the high-luminance part. Therefore, halation of the high-luminance part is minimized.

According to a 24th aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, tone correction is not carried out when the luminance distribution is discrete. Therefore, tone correction is not performed on an image that needs no tone correction.

According to a 25th aspect of the present invention, in the automatic tone correction apparatus according to any of the 1st, 4th, 7th, 10th, and 13th aspects, color-difference signals of the image are corrected according to the correction table. Therefore, such a phenomenon that the saturation appears degraded due to luminance correction is suppressed.

According to a 26th aspect of the present invention, an automatic tone correction method for correcting tones of a digital image comprises: a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value. Therefore, a back-lighted image can be brightly corrected so that the luminance value of a low-luminance region in the back-lighted state is brought to a target luminance value, thereby obtaining an image the whole contrast of which is enhanced. Further, also a forward-lighted image can be corrected so that the luminance value of a low-luminance region is brought to a target luminance value, resulting in an image the whole contrast of which is enhanced.

According to a 27th aspect of the present invention, in the automatic tone correction method according to the 26th aspect, in the tone correction step, a restriction is put on the amount of correction. Therefore, even when the luminance correction target value is not an optimum value, the property of the original image is prevented from being damaged significantly.

According to a 28th aspect of the present invention, in the automatic tone correction method according to the 26th aspect, the luminance correction target value to be used in the tone correction step is previously set according to the position of the center of gravity. Therefore, a work of setting a luminance correction target value for every input image is dispensed with.

According to a 29th aspect of the present invention, an automatic tone correction method for correcting tones of a digital image comprises: a target value calculation step of calculating a luminance correction target value from a target luminance distribution; a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the luminance correction target value that is calculated in the target value calculation step. Therefore, an image having a target brightness and an enhanced contrast as a whole can be obtained using a target luminance distribution of the same shape for a forward-lighted image and a back-lighted image.

According to a 30th aspect of the present invention, in the automatic tone correction method according to the 29th aspect, in the target value calculation step, a target luminance distribution is set with a curve. Therefore, setting of a target luminance distribution is facilitated, whereby the storage capacity is reduced.

According to a 31st aspect of the present invention, in the automatic tone correction method according to the 29th aspect, in the target value calculation step, a plurality of target luminance distributions are stored. Therefore, the luminance correction target value can be easily adjusted.

According to a 32nd aspect of the present invention, an automatic tone correction method for correcting tones of a digital image comprises: a luminance histogram formation step of forming a luminance histogram of the digital image; a boundary value calculation step of calculating a boundary value for setting a range of a low-luminance part of the luminance histogram; a center-of-gravity calculation step of calculating a center of gravity of the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value. Therefore, in a back-lighted image, a low-luminance part including a back-lighted region is correctly calculated, whereby the luminance value of the back-lighted region is corrected to a target luminance value more accurately.

According to a 33rd aspect of the present invention, in the automatic tone correction method according to the 32nd aspect, in the boundary value calculation step, a luminance range is divided into plural regions, and a boundary value is calculated from feature parameters of the respective regions. Therefore, a boundary value that accurately separates the high-luminance side and the low-luminance side is obtained.

According to a 34th aspect of the present invention, in the automatic tone correction method according to the 33rd aspect, in the boundary value calculation step, the luminance range is divided into two regions, and a center of gravity calculated on the lower-luminance side is used as the center of gravity of the low-luminance part of the luminance histogram. Therefore, the tones can be corrected without recalculating the center of gravity of the low-luminance part.

According to a 35th aspect of the present invention, an automatic tone correction method for correcting tones of a digital image comprises: a luminance histogram formation step of forming a luminance histogram of the digital image; a mountain-shape distribution detection step of detecting a range of a mountain-shape distribution of the luminance histogram; a boundary value calculation step of calculating a boundary value for setting a low-luminance part of the luminance histogram, from the range of the mountain-shape distribution detected in the mountain-shape distribution detection step; a center-of-gravity calculation step of calculating a center of gravity or the low-luminance part of the luminance histogram, on the basis of the boundary value calculated in the boundary value calculation step; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches a luminance correction target value. Therefore, when the input image is a back-lighted image, it is corrected so as to have a target brightness and, simultaneously, the whole contrast is enhanced. On the other hand, when the input image is a forward-lighted image, the contrast is enhanced without changing the original brightness.

According to a 36th aspect of the present invention, in the automatic tone correction method according to the 35th aspect, in the boundary value calculation step, the boundary value calculated from the range of the mountain-shape distribution is set by using the luminance histogram. Therefore, a more accurate boundary value can be calculated.

According to a 37th aspect of the present invention, in the automatic tone correction method according to the 35th aspect, in the mountain-shape distribution detection step, a restriction is put on the range of the mountain-shape distribution. Therefore, even when the input image is a forward-lighted image, the brightness of the image can be corrected.

According to a 38th aspect of the present invention, an automatic tone correction method for correcting tones of a digital image comprises: a luminance histogram formation step of forming a luminance histogram of the digital image; a center-of-gravity calculation step of calculating a center of gravity of a low-luminance part of the luminance histogram; a target value correction step of correcting a luminance correction target value so that the target value matches the center of gravity of the low-luminance part of the luminance histogram which is calculated in the center-of-gravity calculation step, when the target value is smaller than the center of gravity of the low-luminance part of the luminance histogram; and a tone correction step of expanding the luminance distribution and, simultaneously, performing tone correction so that the center of gravity of the low-luminance part of the luminance histogram matches the uncorrected luminance correction target value when correction of the target value is not performed, or it matches the corrected luminance correction target value when correction of the target value is performed. Therefore, it is prevented that the corrected image becomes darker than the input image.

According to a 39th aspect of the present invention, in the automatic tone c